Zeolites have well-defined, crystalline structures with pores that are in molecular size range, ion exchangeable sites and high hydrothermal stability. These properties enable zeolites to be widely used as catalysts in petrochemical processing. Nanocrystalline zeolites (with crystal sizes of 100 nm or less) can have advantages over conventional micron sized zeolites in that they have larger external surface areas, shorter diffusion pathlengths and lower tendencies to form coke. The improved properties of nanocrystalline zeolites can lead to new applications in catalysis, environmental protection and chemical sensing. Pure silica nanocrystalline zeolites are promising low-dielectric constant materials for electronics applications. Composite polymer zeolite nanocomposite membranes can be useful in air separation applications.
Current methods to synthesis nanocrystalline zeolites involve terminating the synthesis while the zeolite crystals are still in the nanometer range, thus prohibiting further crystal growth. Such methods synthesize nanocrystalline zeolites at low temperature and ambient pressure which leads to low product yields and long synthesis times. The long synthesis times are a result of the low temperature causing slow nucleation and crystal growth. Typical product yields for the nanocrystalline zeolites are less than 10% of the synthesis gel composition, as compared to near 100% yields for conventional micron-sized zeolites. Once synthesis is complete, nanocrystalline zeolites are present in colloidal suspensions and powder products are then recovered by centrifugation. The remaining synthesis solution is usually discarded after the nanocrystalline zeolites are recovered resulting in adverse environmental effects and the disposal of valuable chemical materials. Previous attempts at reusing the original synthesis solution has required the addition of template species, silicon and/or aluminum sources. In addition, many attempts at reusing the original synthesis solution have failed due to reactants polymerizing and the impurity of the nuclei for subsequent crystal growth.
Selective catalytic reduction (SCR) using ammonia or hydrocarbons is a promising technology for post-combustion treatment of NOx (NO and NO2). Selective catalytic reduction utilizing ammonia (NH3—SCR) has been developed and used worldwide for the control of NOx emissions in fuel combustion from stationary sources due to its efficiency, selectivity and economics. However, NH3 is not a practical reducing agent for NOx emissions from mobile sources due to its toxicity and difficulties in its storage, transportation and handling. A great deal of interest has focused on using urea as a safer source of ammonia in automotive applications. Currently, a solution of urea in water is the preferred choice among different precursors for ammonia. It is generally accepted that urea thermally decomposes in two steps (reaction 1 and 2) to form ammonia and carbon dioxide.(NH2)2CO→NH3+HNCO  (1)HNCO+H2O+NH3+CO2  (2)
Transition metal-containing zeolites have been extensively studied as SCR catalysts. Several recent studies have also used alkali and/or alkaline earth substituted Y zeolites as selective oxidation catalysts and deNOx catalysts. In the absence of transition metals, zeolite Y potentially offers novel SCR pathways different from those that occur in transition metal containing catalysts. Nanocrystalline Y zeolites and metal oxides with particle sizes less than 100 nm, can be particularly useful in environmental applications due to the small crystal sizes and large internal and external surface areas. The influence of intracrystalline diffusion on SCR reaction rates has been previously investigated for transition metal-exchanged zeolites with different crystal sizes.